The present invention relates to a hydraulic control system for an automatic transmission, and more particularly to a hydraulic lock-up control system during a shift taking place in an automatic transmission.
A known hydraulic lock-up control system for an automatic transmission is illustrated on page 33 of a publication entitled "TOYOTA VISTA E-SV10 SERIES A/T MOUNTED CAR, NEW CAR INTRODUCTION MANUAL, JULY 1982." In this known hydraulic system, a lock-up control valve causes a lock-up clutch to be engaged during operation with the fourth speed ratio. The lock-up control valve is shiftable between two positions where the lock-up clutch is caused to be engaged and released, respectively, in response to a hydraulic pressure building up in a servo chamber of a brake to be engaged upon shifting up to the fourth speed ratio and released upon shifting from the fourth speed ratio down to the third speed ratio. More specifically, a lock-up engagement port of the lock-up control valve, namely, a port which, when supplied with a hydraulic pressure, causes a spool of the lock-up control valve to take the position where the lock-up clutch is caused to be engaged, is connected to a hydraulic line through which hydraulic fluid is supplied from a 3-4 shift valve to the servo apply chamber of the above-mentioned fourth speed brake. The hydraulic line is provided with flow restrictor, and the lock-up control valve is connected to the hydraulic line at a section thereof between the servo chamber and the flow restrictor. According to this arrangement, the lock-up clutch is engaged upon shifting from the third speed ratio to the the fourth speed ratio (a 3-4 upshift), and the lock-up clutch released upon shifting from the fourth speed ratio to the third speed ratio (a 4-3 downshift).
This known hydraulic lock-up control system, however, poses a problem that there occurs substantial shock during the 4-3 downshift. This is because the release action of the above-mentioned brake and the release action of the lock-up clutch take place at the same time during the 4-3 downshift, and thus shocks taking place upon release of the brake and shocks taking place upon release of the lock-up clutch are superimposed.
An object of the present invention is to provide a hydraulic control system which is improved such that shocks taking place upon shifting from a n th speed ratio down to a n-1 th speed ratio (where: n is an integer) and upon release of a lock-up device are not superimposed.